Why in some of my fired brass can a bullet easily slide into and others you can not even get to start?Same brass,same bullets,same powder was used in all loads,only difference was different charge weights.

What cartridge and gun?

If it were a crimped neck then a light load might not blow the crimp out as much.

Like Don said, it could be the crimp. At lower pressures brass sometimes springs back, also.

Need more info to be definitive. FWIW, Dutch.

If your necks are getting long they may be intruding into the lead. If so it will put a taper on the end of the mouth. This is dangerious and the cases would need trimming.

The caliber is .220 Swift and all the cases were trimmed to minimum length,and they were all crimped with a Lee factory die.Loads were all around maximum published in Nosler's reloading manual.The crimping may have something to do with it,but the loads did not vary more than 1.5 grains.I have noticed the same thing before in my .243,also using a Lee crimp die.The only reason that I noticed it is I was trying to find my overall length using a dummy cartridge and got a piece of brass that the bullet would not enter.

If the brass has been loaded several times and you had to trim often the case mouth may be getting too thick. In that case you have to ream the case neck or turn the outside. Mike the loaded rounds and check the overall diameter with bullet loaded. My guess is you will find some are larger in diameter. 220 swift is a very high intensity round and if you are loading to the max. as you stated the brass will flow forward quickly. Be careful.